The present invention relates to a control method of an electric vehicle for restricting a slippage of rotating or stationary wheels of the vehicle with respect to a rail road and, particularly, such method by which a reliability of detection of the slippage is improved.
An example of known control methods is disclosed in "Electric Vehicle Science", Journal of Electric Vehicle Research Organization, 1987, March, page 30 (FIG. 15). The control method disclosed therein comprises a detection of rotation frequency of a rotary shaft of an induction motor controlled by a VVVF inverter, i.e., wheels of a vehicle, a generation of a slip signal or glide signal when a differentiation of the rotation frequency exceeds a constant detection level, a reduction of slip frequency which determines a frequency of the VVVF inverter and a restriction of the slippage.
For example, when a driving force given by the induction motor to the wheels during power running control exceeds a maximum driving force (without slippage), the rotation frequency of the induction motor increases abruptly due to slippage. A VVVF inverter control device produces a slip signal when the differentiation of the rotation frequency exceeds a predetermined detection level upon which the slip frequency is reduced to reduce an output torque of the induction motor to thereby recover an optimum friction of the wheels to the rails. Similarly, for a slippage during braking control, the friction of the wheels to the rails is recovered again by reducing the slip frequency to restrict a braking force.
It has been known, however, that the time differentiation of the rotation frequency depends upon rail road condition along which the vehicle runs. For example, when the vehicle runs along a down-hill rail road during power running control, the time differentiation increases. On the contrary, it is reduced when the vehicle runs along an up-hill rail road.
Since the conventional control method of electric vehicle detects a slippage condition on the basis of a constant detection level, there is a case where a non-slip condition is detected as a slippage or an actual slipping condition can not be detected when the vehicle runs along an inclined rail road.